CogniFit's Science blog: Plasticity Unnoticed: Male And Female Brains Change Differentially Across The Lifespan

Plasticity Unnoticed: Male And Female Brains Change Differentially Across The Lifespan

Most scientists believe that the male and female brains are quite similar and differentiated through a genetically determined process: genetic content determine whether the new being will develop ovaries or testes, these, in turn secrete corresponding hormones which operate directly on brain cells to characterize its sex.

A new article, published in Nature Neuroscience, presents an exciting and more complex life-long process that continually acts on the brain to accrue male and female differentiation. Sexual differentiation can occur through interactions and compensatory mechanisms that implicate a large number of genetic, epigenetic, experiential and environmental factors.

If you ask a neuroscientist about how the male brain differs from the female brain, he or she will likely tell you that the sex of the individual has a minor effect on brain structure and organization. He or she will explain that, except for a few differentiated male and female neural circuits, male and female brains are remarkably similar.

They will add that genetic content determine whether the new being will develop ovaries or testes, these, in turn secrete corresponding hormones which operate directly on brain cells to characterize its sex, so that most researchers conceptualize sexual differentiation of the brain as a simple, linear process: "genetic sex determines gonadal sex and gonadal hormones determine brain sex.". You will be impressed by the simplicity of this approach and you will easily see how such a straightforward concept might be translated into research.

Yet an article published by Margaret McCarthy from the University of Maryland; Arthur Arnold, from the University of California, in Nature Neuroscience, last month, discredits this existing approach.

If you ask them, this simple view of brain sexual differentiation exists due to a number of factors. Firstly, they will point out, a majority of available studies are uninformative on sex differentiation of the brain because they do not provide information about the sex of the animal population under study or because they investigate one sex, preferably, males.

Secondly, most research on the sexual differentiation of the brain has dealt with a restricted number of brain functions and brain areas such as ovulation and its corresponding brain area, so that conclusions reached from these limited research fields have incautiously been generalized to the field as a whole.

Mc Carthy and Arnold argue for a new and exciting approach to sex differentiation in the brain. They claim that evidence accumulating from several sources indicates that sexual differentiation of the brain is not the simple linear, time-limited process it is believed to be. These authors claim that brain organization and structure is the result of a multiplicity of sex-specific factors which act in parallel, interact with each other and compensate for each other for as long as an individual lives. The authors describe a number of newly identified factors that play a significant role in sexual brain differentiation of the brain.

Among those are the sex-specific multiple genes in the X and Y chromosomes, not just Sry (a gene on the Y chromosome that determines the sex of the individual), as previously thought. There are factors encoded in the gonadal and non-gonadal chromosomes that signal directly to the brain, and not just the gonadal hormones as previously believed. There are also complex cellular communication mechanisms that are specific to brain regions and have a compensatory role. They mediate sex-specific changes on gonadal hormone levels across the life span and facilitate or inhibit sex-differentiation effects.

Last and not least, there are sex-specific experiential and environmental influences. All these affect brain structure and organization in sex-specific ways. They require more subtle and complex research models to study sexual differentiation of the brain.

This view will have profound implications for 21st century perception of the male and female brains as there will be a need to evolve from a simple view of the sexual brain where all is almost solely determined by genetics, that sets forward a serial developmental process, to a more complex view where sexual differentiation of the brain evolves and changes across the life span in response to a multiplicity of genetic and epigenetic sex-specific factors.

Because we know very little about how those last effects (environment and experience) affect brain biology, we can expect a wealth of studies which will manipulate experience and environment to study and/or control their effect on neuronal and cerebral sexual differentiation.

Some of these studies will no doubt use cognitive training and brain training to explore if rates of developmental conditions, for example learning disabilities which are now higher in males and age-linked neurodegenerative conditions which are now more prevalent in females, can be reversed to affect the cerebral biology of men and women.